(a) Technical Field
The present invention relates to a rotor permanent magnet apparatus for a drive motor based on temperature distribution, in which a coercivity enhancing material is diffused on the surface of a permanent magnet, thereby improving coercivity without the loss of magnetic flux density.
(b) Background Art
In recent years, a permanent magnet type motor has been developed, which uses a magnet having the ability to attract or repel other magnetic materials and the ability to permanently generate a static magnetic field without using external energy.
A physical difference from the magnet of other magnetic materials is that, for the permanent magnet, effective magnetization M remains after removal of an external magnetic field, and magnetization inversion (e.g., demagnetization) is initially induced under application of heat or a relatively large reversal magnetic field, therefore magnetization M decreases.
FIG. 1 is an exemplary simulation diagram showing the temperature distribution of a permanent magnet mounted within a rotor of a drive motor. When the driver motor operates under a continuous load of 4000 rmp, the temperature of a central portion of the permanent magnet mounted within the rotor increases (indicated by a darker shade in FIG. 1). The temperature increases because heat at the central portion of the permanent magnet is not substantially dissipated to the exterior of the permanent magnet. When the temperature at the central portion of the permanent magnet increases as described above, the permanent magnet may be demagnetized.
Moreover, magnets have different coercivities, and the coercivity affects the demagnetization of a magnet. For example, as the coercivity of the magnet increases, it may be difficult to demagnetize the magnet at high temperature. On the other hand, as the coercivity of the magnet decreases, it may be easier to demagnetize the magnet at high temperature. Therefore, a magnet having high coercivity may be used to prevent demagnetization of the permanent magnet.
Conventionally, dysprosium (Dy) was mixed with a neodymium (Nd) magnet to increase the content of Dy, thereby enhancing the coercivity of the magnet. However, when the content of Dy is increased, the coercivity of the magnet may be enhanced, but the remaining magnetic flux density Br decreases. When the content of Dy is increased to enhance the existing coercivity, the content of Dy is constantly applied to the central portion of the permanent magnet, which has high temperature, and both the ends of the permanent magnet, which have low temperature, and therefore, the entire cost of the permanent magnet increases.